Add event dispatch on successful Email TFA (expose TfaSucceedEvent)

Problem/Motivation

The email_tfa module does not currently expose an event when the email-based 2FA workflow completes successfully.
For external integrations (auditing, analytics, custom rate limiting, notifications, SSO flows, etc.), it is useful to have a Symfony event that other modules can subscribe to when Email TFA succeeds—without relying on hooks or overrides.

Steps to reproduce

• Enable email_tfa and configure the standard email verification flow.
• Perform a login that requires Email TFA and complete verification successfully.
• There is no public event fired to indicate “TFA succeed” for other modules to subscribe to.

Proposed resolution

Expose a TfaSucceedEvent and dispatch it right after Email TFA is validated successfully and just before/around user_login_finalize($user).

Remaining tasks

• Confirm the exact dispatch point (after successful Email TFA validation, before/around user_login_finalize() as appropriate).
• Add kernel test(s) verifying:
  • email_tfa.succeed is dispatched when Email TFA completes successfully.
  • The event provides UserInterface and timestamp.
• Update README/CHANGELOG to document the new event.